Servo valve

ABSTRACT

A servo valve has a movable element disposed inside a body, and a drive unit to slide the movable element in the axial direction. A first elastic portion on one end portion side of the body has a first elastic force to press the movable element toward the drive unit connected to another end portion of the body; a second elastic portion on another end portion side of the body has a second elastic force to press the movable element toward the one end portion side of the body. A connecting portion is connected to the second elastic portion, wherein at a neutral position of the movable element, the connecting portion abuts against an inner peripheral surface of the body and against the movable element. An end of one of the first and second elastic portions is fixed directly to the movable element, and an end of the other of the first and second elastic portions is connected to the connecting portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2017-235667 filed on Dec. 8, 2017 andU.S. patent application Ser. No. 15/956,369 filed on Apr. 18, 2018, thecontents all of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a servo valve which switchesconnections of flow passages between a plurality of ports provided in abody by driving a drive unit based on an input signal from the outsideto thereby slide a movable element in the body.

Description of the Related Art

Japanese Patent No. 2859459 (hereinafter referred to as Document 1),Japanese Patent No. 4099749 (hereinafter referred to as Document 2),Japanese Laid-Open Utility Model Publication No. 62-089584 (hereinafterreferred to as Document 3), Japanese Laid-Open Patent Publication No.2006-207796 (hereinafter referred to as Document 4), and ChineseLaid-Open Patent Publication No. 101737525 (hereinafter referred to asDocument 5) each disclose a servo valve for switching connections offlow passages between a plurality of ports provided in a body by drivinga drive unit based on an input signal from the outside to thereby slidea movable element provided in the body in an axial direction of thebody.

Document 1 discloses a servo valve in which a spool (movable element) isslidably housed inside a sleeve arranged in a body. Further, Documents 2to 4 each disclose a servo valve in which a movable element is returnedto a neutral position (a position of the movable element when operationof a drive unit is stopped) by use of mechanical springs. Document 5discloses a servo valve in which a movable element is returned to aneutral position by use of a magnetic spring. Incidentally, when themovable element is returned to the neutral position, the valve is placedin a state of a closed center with all the ports being closed, anexhaust center with output ports and exhaust ports being held incommunication, or a pressure center with output ports and a supply portbeing held in communication.

SUMMARY OF THE INVENTION

However, in the servo valve of Document 1, in an event of occurrence ofan abnormality such as a power failure or the like, the position of themovable element is not fixed because an input signal cannot be suppliedfrom the outside to the drive unit. Thus, there is concern that theservo valve is brought into a valve opened state due to its own weightof the movable element depending on a mounting posture of the servovalve.

Further, in the servo valves of Documents 2 and 3, a restoring force ofthe mechanical spring that returns the movable element to the neutralposition is proportional to a deviation amount from the neutralposition. Thus, in the vicinity of the neutral position, the restoringforce becomes small, so that the movable element is positioned unstably.As a result, a valve opened state may occur due to vibration or the likefrom the outside.

Furthermore, in the servo valve of Document 4, if the mechanical springsand the like have backlash or looseness, it becomes difficult to performpositioning control of the movable element or opening-degree control fora plurality of ports.

In addition, in the servo valves of Documents 2 to 4, in sliding themovable element, a motor (drive unit) with a large thrust force isrequired to overcome an initial load exerted on the movable element bythe mechanical spring with the movable element being at the neutralposition. Further, in the servo valves of Documents 2 to 4, it becomesdifficult to control the positioning of the movable element and tocontrol the opening degrees of the plurality of ports due to variationof a spring force depending on the position of the movable element.

Furthermore, in the servo valve of Document 5, the magnetic attractiveforce of a permanent magnet attached to the movable element is used toreturn the movable element to the neutral position. In this case aswell, since the restoring force for returning the movable element to theneutral position is proportional to a deviation amount from the neutralposition, the restoring force becomes smaller around the neutralposition, so that the movable element is positioned unstably.

The present invention has been made with such problems taken intoconsideration, and it is an object of the present invention to provide aservo valve capable of stably performing positioning control of amovable element.

According to an aspect of the present invention, there is a servo valveincluding a tubular body having a plurality of ports formed therein, amovable element disposed inside the body in an axial direction of thebody, and a drive unit connected to the body in the axial direction andconfigured to slide the movable element in the axial direction tothereby switch connections of flow passages between the ports.

The servo valve is further provided with a first elastic portion, asecond elastic portion and a connecting portion.

The first elastic portion extends in the axial direction inside the bodyand has a first elastic force to press the movable element toward thedrive unit side in the axial direction.

The second elastic portion extends in the axial direction inside thebody and has a second elastic force to press the movable element in adirection away from the drive unit along the axial direction.

The connecting portion is connected to at least the second elasticportion inside the body, and abuts against a portion of the body thatfaces the drive unit and a portion of the movable element that faces thedrive unit, at a neutral position of the movable element at whichdriving of the drive unit is stopped.

As described above, the first elastic portion and the second elasticportion have elastic forces (the first applied in mutually differentdirections along the axial direction.

In this case, at the neutral position, the connecting portion is pressedagainst a portion of the body that faces the drive unit and a portion ofthe movable element that faces the drive unit, by the second elasticforce. Thus, since the connecting portion is restrained from moving in adirection away from the drive unit, the position of the second elasticportion is restrained between the drive unit side and the opposite sideof the drive unit inside the body. As a result, since the second elasticforce is not exerted on the movable element, the movable element ispositioned at the neutral position where the movable element abutsagainst the connecting portion.

Next, when the movable element is slid toward the drive unit by drivingof the drive unit, the movable element is slid together with theconnecting portion toward the drive unit in the axial direction againstthe second elastic force. In this case, when the driving of the driveunit is discontinued, the second elastic force serves as a restoringforce, so that the connecting portion and the movable element arereturned to the neutral position in the axial direction.

On the other hand, when the movable element is slid in a direction awayfrom the drive unit by driving of the drive unit, the movable element isslid in a direction away from the drive unit of the axial directionagainst the first elastic force in a state that the connecting portionis in abutment against a portion of the body that faces the drive unit.In this case, when the driving of the drive unit is discontinued, thefirst elastic force serves as a restoring force, so that the movableelement is returned to the neutral position in the axial direction.

Accordingly, in the present invention, in any of the case that themovable element moves toward the drive unit and the case that themovable element moves away from the drive unit, it is possible to stablyperform the positioning control of the movable element relative to theneutral position (i.e., the opening control of the respective ports). Asa result, it is possible to realize a servo valve having a satisfactoryfunction of closed center, exhaust center or pressure center.

In this case, one end of the first elastic portion may be fixed to thedrive unit side of the movable element, one end of the second elasticportion may be fixed to the drive unit side of the body, and the otherend of the first elastic portion and the other end of the second elasticportion may be connected to the connecting portion. Thus, with a simplestructure, it is possible to improve the controllability in positioningthe movable element with respect to the neutral position.

Herein, the drive unit has a tubular body containing a magnetic body andconnected to the body in the axial direction, and a movable portionprovided inside the tubular body and forming a portion of the movableelement, the movable portion including a movable magnet, a movable coilor a movable iron core. With this structure, the movable portion ismoved in the axial direction, whereby the movable element including themovable portion can be slid in the axial direction. Thus, regardless ofthe type of the movable portion, i.e., in any of a movable magnet type,a movable coil type and a movable iron core type, it is possible toimprove the controllability in positioning the movable element.

Further, in the servo valve, it is possible to adjust a restoring forcefor returning the movable element to the neutral position by balancing aforce exerted on the movable element from the drive unit with the firstelastic force or the second elastic force. Thus, it is possible toimprove the positioning control of the movable element.

Incidentally, by adjusting the restoring force such that the restoringforce is constant irrespective of the position of the movable element inthe axial direction, it is possible to further improve the positioningcontrol of the movable element. In this case, if the force exerted onthe movable element is a magnetic attractive force generated at themovable portion, it is possible to make the restoring force constant bybalancing the magnetic attractive force with the first elastic force orthe second elastic force.

Here, description will be made regarding the configuration of the servovalve where the movable portion is of the movable magnet type.

The drive unit has a first yoke, which serves as the tubular body,connected to the body in the axial direction, a coil wound around thefirst yoke, and a magnet portion, which serves as the movable portion,provided inside the first yoke so as to face the coil. In this case, bya magnetic attractive force exerted on the magnet portion due toenergization to the coil, the movable element is slid in the axialdirection.

That is, magnetic flux is generated around the magnet portion byenergization to the coil, and thus, by the magnetic attractive forcearising from the magnetic flux, it is possible to slide the movableelement including the magnet portion in the axial direction against theelastic force of the first elastic portion or the second elasticportion. That is, the drive unit functions as a linear motor for movingthe magnet portion in the axial direction. Thus, since it is possible toeasily perform the positioning control of the movable element, it ispossible to improve the responsiveness of the servo valve to an inputsignal supplied from the outside to the coil.

Further, protruding portions protruding inward of the first yoke may beprovided respectively at one end side and the other end side of thefirst yoke in the axial direction. In this case, at the neutral positionat which energization to the coil is stopped, the magnet portion ispositioned between the two protruding portions.

With this structure, since the protruding portions each constitute apart of a magnetic path of the magnetic flux when the coil is energized,the magnetic attractive force can be increased as the magnet portioncomes closer to the protruding portion by movement of the magnet portionin the axial direction. Further, the magnetic attractive force isbalanced with the first elastic force or the second elastic force tothereby adjust the restoring force, and thus, it is possible to furtherimprove the controllability of the servo valve (the positioning controlof the movable element and the responsiveness of the servo valve).

In this case, the coil may be provided between the two protrudingportions inside the first yoke, and when the movable element is at theneutral position, the magnet portion and the coil may be located atsubstantially the same position in the axial direction. Thus, it ispossible to further improve the controllability of the servo valve.

Further, in the servo valve, the first yoke may be connected to the bodyso as to cover the magnet portion within a moving range within which themagnet portion is moved in the axial direction by sliding of the movableelement. Thus, it is possible to further improve the controllability ofthe servo valve.

Further, the first yoke may be composed of two yokes arranged so as tointerpose the coil therebetween in the axial direction. With thisstructure, the assembling performance of the servo valve can beimproved.

It is preferable that the magnet portion should contain two permanentmagnets arranged in the axial direction and magnetized in the axialdirection, and a second yoke interposed between the two permanentmagnets. With this structure, since at the time of energization to thecoil, the magnetic flux generated around the magnet portion passesthrough the second yoke, a large thrust force arising from the magneticattractive force is generated at the magnet portion in the axialdirection. Therefore, it is possible to easily slide the movable elementin the axial direction against the first elastic force or the secondelastic force.

In this case, the two permanent magnets may be magnetized in mutuallydifferent magnetization directions. Thus, it is possible to easily slidethe movable element toward the drive unit or away from the drive unit inthe axial direction.

Further, the aforementioned servo valve may specifically be constructedas described below. That is, a sleeve provided with openingscommunicating with the respective ports is disposed inside the body. Inthis case, the movable element has the magnet portion, a spool disposedinside the sleeve the axial direction, a shaft connecting the magnetportion and the spool in the axial direction, and an annular first fixedportion disposed on the magnet portion side of the shaft, one end of thefirst elastic portion being fixed to the first fixed portion.

Thus, an annular second fixed portion may be provided inside the bodyand on the first yoke side, the second fixed portion being fixed to thebody and the first yoke, wherein the shaft and the first fixed portionmay penetrate through the second fixed portion, and one end of thesecond elastic portion may be fixed to the second fixed portion.Further, the connecting portion may be an annular member configured to,inside the body, abut against the spool and a portion of the body thatis located on the spool side, the shaft penetrating through theconnecting portion.

Then, the first elastic portion may be interposed between the connectingportion and the first fixed portion inside the body, and the secondelastic portion may be interposed between the connecting portion and thesecond fixed portion inside the body.

Further, the servo valve may be further provided with a sensor disposedadjacent to the magnet portion in the axial direction and configured todetect magnetic flux. With this structure, it is possible to easilygrasp the position of the movable element relative to the neutralposition from variation in the magnetic flux detected by the sensor.Consequently, it is possible to perform a suitable servo control byadjusting an input signal supplied to the coil depending on the positionof the movable element.

Further, in a case that the movable portion is of the movable coil type,the servo valve may be constructed as described below. That is, thedrive unit has a yoke, which serves as the tubular body, connected tothe body in the axial direction, two permanent magnets providedrespectively at opposite ends of the yoke in the axial direction, aniron core provided inside the yoke so as to face the yoke, and a coilwound around the iron core. In this case, the movable portion containsthe iron core and the coil, and the movable element is slid in the axialdirection by a magnetic attractive force including at least one of aforce acting between the two permanent magnets and the iron core and aforce exerted on the movable portion due to energization to the coil.

On the other hand, in a case that the movable portion is of the movableiron core type, the servo valve may be constructed as described below.That is, the drive unit has a yoke, which serves as the tubular body,connected to the body in the axial direction, a permanent magnetprovided at a center portion of the yoke in the axial direction, a coilprovided inside the yoke in the axial direction so as to face thepermanent magnet, and an iron core, which serves as the movable portion,provided inside the yoke in the axial direction. In this case, themovable element is slid in the axial direction by a magnetic attractiveforce including at least one of a force acting between opposite ends ofthe yoke and the iron core and a force exerted on the iron core due toenergization to the coil.

In any of the cases of the movable coil type and the movable iron coretype, as in the case of the movable magnetic type, the movable elementcan be slid in the axial direction by the magnetic attractive force.Therefore, it is possible to easily perform the positioning control ofthe movable element. As a result, it is possible to improve theresponsiveness of the servo valve.

Further, in the servo valve, one end of the first elastic portion may befixed to the movable element on a side that is across from the driveunit, the other end of the first elastic portion may be fixed to an endcover configured to close an end of the body that is located across fromthe drive unit, one end of the second elastic portion may be fixed to aportion of the body that is located on the drive unit side, and theother end of the second elastic portion may be connected to theconnecting portion. In this case as well, with a simple structure, it ispossible to improve the controllability in positioning the movableelement relative to the neutral position.

Incidentally, the first elastic portion and the second elastic portionmay be spring members. Thus, it is possible to reduce cost of the servovalve.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings, in which apreferred embodiment and several modifications of the present inventionare shown by way of illustrative examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a servo valve according to anembodiment of the present invention;

FIG. 2A to FIG. 2C are schematic diagrams each showing the movement of amovable element shown in FIG. 1;

FIG. 3A to FIG. 3C are schematic diagrams each showing the movement of amagnet portion shown in FIG. 1;

FIG. 4 is a graph showing a relationship between the position of themovable element shown in FIG. 1 and the force exerted on the movableelement;

FIG. 5 is a graph showing a relationship between the position of a spooland thrust force;

FIG. 6 is a graph showing a relationship between magnetic flux densitydetected by a magnetic sensor shown in FIG. 1 and the position of themovable element;

FIG. 7A to FIG. 7C are schematic diagrams each showing the movement of amovable element in a servo valve of Document 3;

FIG. 8 is a graph showing a relationship between the position of themovable element and the force exerted on the movable element in theservo valve of Document 3;

FIG. 9A to FIG. 9C are schematic diagrams showing the movement of amovable element in a servo valve of Document 4;

FIG. 10A and FIG. 10B are schematic diagrams each showing the movementof the movable element in the servo valve of Document 4;

FIG. 11 is a graph showing a relationship between the position of themovable element and the force exerted on the movable element in theservo valve of Document 4;

FIG. 12 is a graph showing a relationship between the position of amovable element and the force exerted on the movable element in a servovalve of Document 5;

FIG. 13 is a graph comparing relationships between the positions of themovable elements and the forces exerted on the movable elements in theservo valve according to the present embodiment and the servo valves ofDocuments 3 to 5;

FIG. 14A to FIG. 14C are schematic diagrams each showing the movement ofa movable portion of a movable coil type;

FIG. 15A to FIG. 15C are schematic diagrams each showing the movement ofa movable portion of a movable iron core type;

FIG. 16 is a cross sectional view of a servo valve according to amodification; and

FIG. 17 is a graph showing a relationship between the position of amovable element shown in FIG. 16 and the force exerted on the movableelement.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, a preferred embodiment of a servo valve according to thepresent invention will be described in detail with reference to theaccompanying drawings.

1. CONFIGURATION OF SERVO VALVE 10

FIG. 1 is a cross sectional view of a servo valve 10 according to thepresent embodiment.

The servo valve 10 is equipped with a tubular valve body 12, a movableelement 16 disposed inside the valve body 12 substantially coaxiallywith a central axis 14 of the valve body 12, and a drive unit 18connected to the valve body 12 in a direction (axial direction) alongthe central axis 14 and configured to slide the movable element 16 inthe axial direction inside the valve body 12. Incidentally, the centralaxis 14 is a central axis along the longitudinal direction of the servovalve 10 including the valve body 12. In the following description, adirection (the axial direction) along the central axis 14 will bereferred to as an X direction, wherein a direction toward the left sidein FIG. 1 (the direction toward the valve body 12 side of the servovalve 10) will be referred to as an X1 direction, while a directiontoward the right side in FIG. 1 (the direction toward the drive unit 18side of the servo valve 10) is referred to as an X2 direction.

The valve body 12 is a tubular body in which a hole portion 20penetrating in the X direction and accommodating the movable element 16is formed substantially coaxially with the central axis 14. The holeportion 20 is a stepped through hole composed of two large diameterportions 20 a, 20 b respectively formed on one end side of the valvebody 12 toward the X1 direction and on the other end side of the valvebody 12 toward the X2 direction, and a central small diameter portion 20c connecting the two large diameter portions 20 a, 20 b. An end cover 22is attached to one end of the valve body 12 on the X1 direction side soas to close the hole portion 20. On the other hand, the drive unit 18 isconnected to the other end of the valve body 12 on the X2 directionside.

The valve body 12 has a plurality of ports 24 formed in an outerperipheral surface thereof, the ports 24 radially communicating with thesmall diameter portion 20 c of the hole portion 20. In FIG. 1, fiveports 24 are formed. That is, the servo valve 10 is a five-port servovalve which controls flow direction of fluid such as air or the like bysliding the movable element 16 in the X direction and thereby switchingthe connections of flow passages between the five ports 24.Incidentally, the number of the ports 24 may be suitably set dependingon the specification of the servo valve 10.

A tubular sleeve 28 having a plurality of openings 26 communicating withthe plurality of ports 24 is disposed at the small diameter portion 20 cof the hole portion 20 in contact with an inner peripheral surface ofthe valve body 12.

The drive unit 18 has a tubular first yoke 32 connected to the other endof the valve body 12 on the X2 direction side, a coil 34 wound aroundthe first yoke 32, and a magnet portion (movable portion) 36 providedinside the first yoke 32 so as to face the coil 34. The first yoke 32 ismade of a magnetic body and has a hole portion 38 formed substantiallycoaxially with the central axis 14. The hole portion 38 of the firstyoke 32 penetrates in the X direction to communicate with the holeportion 20 of the valve body 12 and houses the magnet portion 36therein. As described later, the magnet portion 36 is constituted as aportion of the movable element 16. Thus, the magnet portion 36 is amovable portion of a movable magnet type. Further, the first yoke 32 isconnected to the valve body 12 so as to cover a moving range withinwhich the magnet portion 36 is moved in the X direction by slidingmovement of the movable element 16.

The first yoke 32 is a yoke having a divided structure composed of aside yoke 32 a connected to the other end of the valve body 12 on the X2direction side, and an outer yoke 32 b connected to an X2 direction sideof the side yoke 32 a. The side yoke 32 a is a tubular body constitutingone end portion of the first yoke 32 on the X1 direction side and havinga first protruding portion 40 a protruding toward the hole portion 38.The outer yoke 32 b is a tubular body constituting the other end portionof the first yoke 32 on the X2 direction side and having a secondprotruding portion 40 b protruding toward the hole portion 38. The coil34 is formed by winding a conductive wire around a bobbin 42 made of anelectrical insulating material. The coil 34 is disposed between thefirst protruding portion 40 a and the second protruding portion 40 binside the first yoke 32 so as to face the magnet portion 36.

An end cover 46 made of a non-magnetic body is attached to the other endof the first yoke 32 (the outer yoke 32 b) on the X2 direction side soas to close the hole portions 20, 38. A magnetic sensor 48 for measuringmagnetic flux density is disposed in the end cover 46 substantiallycoaxially with the central axis 14.

The magnet portion 36 is composed of an annular first permanent magnet36 a disposed on the X1 direction side, an annular second permanentmagnet 36 b disposed on the X2 direction side, and an annular secondyoke 36 c as a center yoke made of a magnetic body interposed betweenthe first permanent magnet 36 a and the second permanent magnet 36 b.The first permanent magnet 36 a and the second permanent magnet 36 b aremagnetized in different directions from each other along the Xdirection. That is, the first permanent magnet 36 a is magnetized suchthat the X1 direction side thereof is N-pole while the X2 direction sidethereof is S-pole. The second permanent magnet 36 b is magnetized suchthat the X1 direction side thereof is S-pole while the X2 direction sideis N-pole. Incidentally, in the present embodiment, the aforementionedmagnetizing direction is one example, and the magnetized direction maybe any direction as long as the first permanent magnet 36 a and thesecond permanent magnet 36 b are magnetized in mutually differentdirections along the X direction.

The first permanent magnet 36 a, the second yoke 36 c and the secondpermanent magnet 36 b are connected on a non-magnetic connector shaft 50which extends in the X direction substantially coaxially with thecentral axis 14. Accordingly, the magnet portion 36 is disposed in thehole portion 38 substantially coaxially with the central axis 14.

In the energized state that an input signal is supplied from the outsideto the coil 34, the movable element 16 is slid inside the hole portions20, 38 in the X direction (toward the X1 direction or X2 direction) by aforce exerted on the magnet portion 36, that is, by a thrust forcegenerated at the magnet portion 36 by a magnetic attractive forcearising from magnetic flux which is generated around the magnet portion36 by energization to the coil 34. FIG. 1 shows the position of themovable element 16 when driving of the drive unit 18 is stopped, thatis, supply of an input signal to the coil 34 (energization to the coil34) is stopped. In the following description, the position at this timeis referred to as a neutral position of the movable element 16.

Further, in the following description, the magnetic attractive force isan all-inclusive term of forces that act on the magnet portion 36 due tothe magnetic flux generated around the magnet portion 36. Thus, themagnetic attractive force also includes a force arising from magneticfluxes from the first permanent magnet 36 a and the second permanentmagnet 36 b, and a force exerted on the magnet portion 36 due toenergization to the coil 34.

The movable element 16 has the connector shaft 50 extendingsubstantially coaxially with the central axis 14 in the X direction, themagnet portion 36 connected to the X2 direction side of the connectorshaft 50, a spool 54 disposed in the sleeve 28 along the X directionsubstantially coaxially with the central axis 14 and connected to the X1direction side of the connector shaft 50, and an annular first fixedportion 56 disposed on the magnet portion 36 side of the connector shaft50. The first fixed portion 56 is fixed to the magnet portion 36 and theconnector shaft 50.

An annular connecting portion 58 through which the connector shaft 50penetrates is disposed movably in the X direction in a large diameterportion 20 b of the hole portion 20 on the X2 direction side. Further,in the large diameter portion 20 b, there is provided an annular secondfixed portion 60 through which the connector shaft 50 and the firstfixed portion 56 penetrate and which is fixed to an inner peripheralsurface of the valve body 12 and an end portion of the side yoke 32 a onthe X1 direction side.

Further, the large diameter portion 20 b is provided with a firstelastic portion 62 which has one end fixed to the first fixed portion 56and the other end connected to the connecting portion 58. The firstelastic portion 62 is a spring member such as a compression coil springor the like extending in the X direction between the first fixed portion56 and the connecting portion 58 so as to surround the connector shaft50, and has a first elastic force acting on the drive unit 18 side inthe X2 direction. That is, the first elastic portion 62 in a compressedstate in the X direction is interposed between the first fixed portion56 and the connecting portion 58, whereby the first elastic force isgenerated to press the movable element 16 including the first fixedportion 56 toward the X2 direction.

Still furthermore, the large diameter portion 20 b is provided with asecond elastic portion 64 which has one end fixed to the second fixedportion 60 and the other end connected to the connecting portion 58. Thesecond elastic portion 64 is a spring member such as a compression coilspring or the like extending in the X direction between the second fixedportion 60 and the connecting portion 58 so as to surround the connectorshaft 50 and the first elastic portion 62, and has a second elasticforce acting toward the X1 direction away from the drive unit 18. Thatis, the second elastic portion 64 in a compressed state in the Xdirection is interposed between the second fixed portion 60 and theconnecting portion 58, whereby the second elastic force is generated topress the connecting portion 58 toward the X1 direction.

In this way, the first elastic portion 62 and the second elastic portion64 are arranged so that the direction (X2 direction) in which the firstelastic force acts and the direction (X1 direction) in which the secondelastic force acts are mutually different from each other. Incidentally,in the present embodiment, the first elastic portion 62 and the secondelastic portion 64 may be arbitrarily arranged as long as they haveelastic forces acting in mutually different directions along the Xdirection. Further, the connecting portion 58 and the first fixedportion 56 function as spring seats (spring guides) for the firstelastic portion 62. Furthermore, the connecting portion 58 and thesecond fixed portion 60 function as spring seats (spring guides) for thesecond elastic portion 64.

As mentioned above, FIG. 1 shows the state of the servo valve 10 whenthe movable element 16 is at the neutral position. At this neutralposition, by the second elastic force of the second elastic portion 64,the connecting portion 58 is held in abutment against a step portion 66between the large diameter portion 20 b and the small diameter portion20 c of the hole portion 20 on the inner peripheral surface of the valvebody 12, an end portion of the sleeve 28 on the X2 direction side, andan end portion of the spool 54 on the X2 direction side. However, sincethe connecting portion 58 abuts against the step portion 66 and therebyis restrained from moving in the X1 direction, the second elastic forceis not exerted on the sleeve 28 and the spool 54.

Further, at the neutral position, the first elastic force of the firstelastic portion 62 acts on the first fixed portion 56. However, if thefirst elastic force and the second elastic force are adjusted to balancewith each other, the load imposed on the movable element 16 becomes zeroin total. Accordingly, in the following description, the forces (thefirst elastic force and the second elastic force) exerted on the movableelement 16 at the neutral position are also referred to as an initialload.

Furthermore, at the neutral position, the spool 54 blocks thecommunications between the ports 24 and the hole portion 20 (theconnections of flow passages between the ports 24). That is, the servovalve 10 shown in FIG. 1 is a servo valve having the function of aclosed center. Further, the magnet portion 36 is located between the twoprotruding portions 40 a, 40 b. That is, the position of the second yoke36 c, which is a center position of the magnet portion 36 in the Xdirection, and the center position of the coil 34 in the X direction aresubstantially the same position.

2. OPERATION OF SERVO VALVE 10

The operation of the servo valve 10 as constructed above will bedescribed with reference to FIG. 2A to FIG. 6. Incidentally, thisdescription of operation will be made with reference also to FIG. 1 asnecessary.

Here, description will be made regarding a case that the movable element16 is slid toward the X2 direction as shown in FIG. 2B and FIG. 35 fromthe neutral position shown in FIG. 2A and FIG. 3A and a case that themovable element 16 is slid toward the X1 direction as shown in FIG. 2Cand FIG. 3C. Incidentally, FIG. 2A to FIG. 2C are schematic diagramseach schematically illustrating the operation of the movable element 16disposed in the hole portion 20 of the valve body 12. FIG. 3A to FIG. 3Care schematic diagrams each schematically illustrating the operation ofthe magnet portion 36 constituting the movable element 16.

First of all, at the neutral position shown in FIG. 2A and FIG. 3A, theconnecting portion 58 is pressed by the second elastic force of thesecond elastic portion 64 against the step portion 66 inside the valvebody 12. Thus, movement of the connecting portion 58 toward the X1direction is restrained, and the position of the second elastic portion64 is regulated in the large diameter portion 20 b. As a result, evenwhen the connecting portion 58 abuts against the movable element 16 (thespool 54 shown in FIG. 1), the second elastic force is not exerted onthe movable element 16. Further, if the first elastic force and thesecond elastic force are adjusted to be in balance with each other atthe neutral position, an initial load exerted on the movable element 16becomes zero in total.

On the other hand, as schematically shown in FIG. 3A, at the neutralposition, the second yoke 36 c of the magnet portion 36 and the coil 34are disposed at substantially the same position in the X direction.Thus, with respect to this position, the magnet portion 36, the coil 34and the first yoke 32 are arranged substantially symmetrically. Thus,the first permanent magnet 36 a faces the first protruding portion 40 a,and the second permanent magnet 36 b faces the second protruding portion40 b.

As a result, as indicated by the black arrows in FIG. 3A, the magneticattractive force is generated from the first permanent magnet 36 a tothe first protruding portion 40 a due to the magnetic flux of the firstpermanent magnet 36 a, and the magnetic attractive force is generatedfrom the second permanent magnet 36 b to the second protruding portion40 b due to the magnetic flux of the second permanent magnet 36 b.

However, as mentioned before, because the magnet portion 36, the coil 34and the first yoke 32 are symmetrically arranged, these magneticattractive forces are in balance with each other, and the movableelement 16 including the magnet portion 36 does not move in the Xdirection. As a result, as shown in FIG. 2A, the movable element 16 ispositioned in a state that the connecting portion 58 is in abutmentagainst the X1 direction side (the spool 54 shown in FIG. 1) of themovable element 16. Thus, at the neutral position, as shown in FIG. 1,the spool 54 is able to block the communications between the pluralityof ports 24 and the hole portion 20 (the connections of flow passagesbetween the ports 24).

Next, description will be made regarding a case that the coil 34 issupplied with an input signal from the outside to thereby be broughtinto the energized state whereby the movable element 16 is slid towardthe X2 direction as shown in FIG. 2B and FIG. 3B.

In this case, since an electric current based on the input signal flowsacross the coil 34, magnetic flux is generated around the magnet portion36. This magnetic flux forms a magnetic path passing through the firstyoke 32, the second yoke 36 c and the like to thereby magnetize thefirst protruding portion 40 a to N-pole and the second protrudingportion 40 b to S-pole. Thus, a repulsive force is generated between thefirst protruding portion 40 a and the first permanent magnet 36 a, whilea magnetic attractive force indicated by the black arrow is generatedbetween the second protruding portion 40 b and the second permanentmagnet 36 b. As a result, due to the magnetic attractive force and therepulsive force, a thrust force toward the X2 direction indicated by theoutlined arrow is generated on the magnet portion 36 (the second yoke 36c). Accordingly, as shown in FIG. 2B, the movable element 16 is able toslide toward the X2 direction together with the connecting portion 58against the second elastic force of the second elastic portion 64 towardthe X1 direction.

On the other hand, when the energization to the coil 34 is discontinuedin a case that the movable element 16 is moved toward the X2 direction,the magnetic attractive force by the energization to the coil 34disappears, and consequently the thrust force becomes zero. As a result,the second elastic force functions as a restoring force toward theneutral position, whereby the movable element 16 and the connectingportion 58 are returned to the neutral position shown in FIG. 2A alongthe X1 direction.

Next, description will be made regarding a case that the coil 34 isenergized to slide the movable element 16 toward the X1 direction asshown in FIG. 2C and FIG. 3C.

Also in this case, since an electric current based on an input signalflows across the coil 34, magnetic flux is generated around the magnetportion 36. The magnetic flux forms a magnetic path passing through thefirst yoke 32, the second yoke 36 c and the like to thereby magnetizethe first protruding portion 40 a to S-pole and the second protrudingportion 40 b to N-pole. Thus, a magnetic attractive force indicated bythe black arrow is generated between the first protruding portion 40 aand the first permanent magnet 36 a, while a repulsive force isgenerated between the second protruding portion 40 b and the secondpermanent magnet 36 b. As a result, due to the magnetic attractive forceand the repulsive force, a thrust force toward the X1 directionindicated by the outlined arrow is generated on the magnet portion 36(the second yoke 36 c). Accordingly, as shown in FIG. 2C, the movableelement 16 is able to slide toward the X1 direction against the firstelastic force of the first elastic portion 62 toward the X2 direction.Incidentally, since the connecting portion 58 is in abutment against thestep portion 66, the movable element 16 slides alone toward the X1direction.

On the other hand, when the energization to the coil 34 is discontinuedin a case that the movable element 16 is moved toward the X1 direction,the magnetic attractive force due to the energization to the coil 34disappears, the thrust force becomes zero. As a result, the firstelastic force functions as a restoring force toward the neutralposition, whereby the movable element 16 is returned to the neutralposition shown in FIG. 2A along the X2 direction.

FIG. 4 is a graph illustrating a relationship between the position ofthe movable element 16 and the force exerted on the movable element 16.The horizontal axis represents the position of the movable element 16.In this case, the neutral position shown in FIG. 1 and FIG. 2A is set tozero, and the X2 direction side and the X1 direction side relative tothe neutral position (0) are defined respectively as a positivedirection and a negative direction. Furthermore, the vertical axisrepresents the force exerted on the movable element 16. In this case,the force exerted on the movable element 16 in the X2 direction isdefined as the force in the positive direction, while the force exertedin the X1 direction is defined as the force in the negative direction.

In the case shown in FIG. 3B and FIG. 30, the magnetic attractive forceindicated by the broken line in FIG. 4 is exerted on the magnet portion36. Here, a magnetic attractive force which causes the movable element16 including the magnet portion 36 to slide toward the positivedirection (X2 direction) is generated in the case shown in FIG. 3B. Onthe other hand, a magnetic attractive force which causes the movableelement 16 to slide toward the negative direction (X1 direction) isgenerated in the case shown in FIG. 30.

Further, as shown in FIG. 3B, when the magnet portion 36 is moved towardthe X2 direction from the position of the magnet portion 36 at theneutral position, the magnetic attractive force becomes larger as thesecond permanent magnet 36 b comes closer to the second protrudingportion 40 b. On the other hand, as shown in FIG. 30, when the magnetportion 36 is moved toward the X1 direction from the position of themagnet portion 36 at the neutral position, the magnetic attractive forcebecomes larger as the first permanent magnet 36 a comes closer to thefirst protrusion portion 40 a.

Therefore, as shown in FIG. 4, the magnetic attractive force becomeslarger from zero toward the positive direction as the movable element 16moves further from the neutral position (0) toward the positivedirection (X2 direction). On the other hand, the magnetic attractiveforce becomes larger from zero toward the negative direction as themovable element 16 moves further from the neutral position (0) towardthe negative direction (X1 direction). Consequently, the magneticattractive force becomes linearly larger toward the positive directionor the negative direction in proportion to a deviation amount from theneutral position.

Further, the first elastic force and the second elastic force shown inFIG. 2A to FIG. 2C are exerted on the movable element 16. In FIG. 4, theelastic force (spring force) which acts from the first elastic portion62 and the second elastic portion 64 on the movable element 16 is shownby a dot-and-dash line.

In the case of FIG. 2B, as the movable element 16 slides toward thepositive direction (X2 direction), the second elastic force which isexerted on the movable element 16 toward the negative direction (X1direction) indicated by the black arrow in FIG. 2B becomes larger. Onthe other hand, in the case of FIG. 2C, as the movable element 16 slidestoward the negative direction (X1 direction), the first elastic forcewhich is exerted on the movable element 16 toward the positive direction(X2 direction) indicated by the black arrow in FIG. 2C becomes larger.

Incidentally, at the neutral position shown in FIG. 2A, the firstelastic portion 62 presses the X2 direction side of the movable element16 toward the X2 direction by the first elastic force, while the secondelastic portion 64 presses the X1 direction side of the movable element16 toward the X1 direction by the second elastic force through theconnecting portion 58. As a result, as shown in FIG. 4, at the neutralposition (0), because an initial load (the first elastic force) towardthe positive direction balances with an initial load (the second elasticforce) toward the negative direction, the movable element 16 is placedin a no-load state in which no load is exerted on the movable element 16in total.

Then, as the movable element 16 moves from the neutral position (0)toward the positive direction (X2 direction), the spring force isincreased by the second elastic force from the initial load toward thenegative direction. On the other hand, as the movable element 16 movesfrom the neutral position (0) toward the negative direction (X1direction), the spring force is increased by the first elastic forcetoward the positive direction from the initial load toward the positivedirection. That is, the spring force increases toward the positivedirection or the negative direction in proportion to a deviation amountfrom the neutral position.

Therefore, in the servo valve 10 according to the present embodiment,the restoring force (the force for return the valve to the neutralposition) exerted on the movable element 16 can be adjusted by balancingthe magnetic attractive force with the mechanical spring force.Specifically, as shown by the solid line in FIG. 4, it is preferable toset the restoring force to a fixed value regardless of the position ofthe movable element 16 by balancing the magnetic attractive force withthe mechanical spring force. Thus, it is possible to improve thecontrollability of the position of the movable element 16.

FIG. 5 shows a relationship between the thrust force generated on themagnet portion 36 and the position of the spool 54 (the movable element16). Incidentally, the horizontal axis represents the position of thespool 54 and as in FIG. 4, the X2 direction is defined as the positivedirection, while the X1 direction is defined as the negative direction.The vertical axis represents the thrust force generated on the movableelement 16 including the magnet portion 36 and as in FIG. 4, the X2direction is defined as the positive direction, while the X1 directionis defined as the negative direction.

As mentioned above, because the initial load toward the positivedirection or the negative direction exists, it is necessary to generatea thrust force exceeding the initial load in sliding the movable element16.

To this end, in the servo valve 10 according to the present embodiment,as shown in FIG. 1 and FIG. 3A to FIG. 3C, the first protruding portion40 a and the second protruding portion 40 b are provided to facerespectively the first permanent magnet 36 a and the second permanentmagnet 36 b, and thus, the magnetic attractive force becomes larger asthe first permanent magnet 36 a comes closer to the first protrudingportion 40 a or as the second permanent magnet 36 b comes closer to thesecond protruding portion 40 b.

Therefore, as shown by the solid line in FIG. 5, it is possible toeasily generate a thrust force capable of moving the movable element 16from the neutral position toward the X1 direction or the X2 directionagainst the initial load. Incidentally, the characteristic of the thrustforce shown in FIG. 5 has a characteristic that is symmetrical to thecharacteristic of the spring force indicated by the dot-and-dash line inFIG. 4 with respect to the neutral position (0). Further, in FIG. 5, thecharacteristic of the thrust force in the servo valves of Document 1 andDocument 2 is shown by the dotted line as a comparative example.

FIG. 6 is a graph showing a relationship between the magnetic fluxdensity detected by the magnetic sensor 48 (refer to FIG. 1) and theposition of the movable element 16. Incidentally, FIG. 6 shows as oneexample a case that the movable element 16 (the magnet portion 36)slides from the neutral position toward the X1 direction.

The magnetic sensor 48 is able to detect magnetic flux density andoutput a detection signal corresponding to the detected magnetic fluxdensity to the outside regardless of the presence or absence of theenergization to the coil 34. In the servo valve 10, no magnetic bodyexists between the magnet portion 36 and the magnetic sensor 48. Thus,if the relationship between the magnetic flux density detected by themagnetic sensor 48 and the position of the movable element 16 (themagnet portion 36) is measured in advance, the position of the movableelement 16 can easily be grasped from the magnetic flux detected by themagnetic sensor 48 when the movable element 16 is actually slid in the Xdirection relative to the neutral position by the energization to thecoil 34, and hence, it is possible to control the input signal suppliedto the coil 34 depending on the position of the movable element 16.

COMPARISON OF PRESENT EMBODIMENT WITH DOCUMENTS 3 TO 5

Next, with reference to FIG. 7A to FIG. 13, the characteristic of theservo valve 10 according to the present embodiment will be compared withcharacteristics of the servo valves in Documents 3 to 5. Here, the samecomponents as those in the servo valve 10 will be denoted by the samereference numerals.

FIG. 7A to FIG. 8 show a case of the servo valve in Document 3.

FIG. 7A to FIG. 7C schematically show the construction of this servovalve. FIG. 7A shows a neutral position, FIG. 7B shows a case that amovable element 16 is moved toward the X2 direction, and FIG. 7C shows acase that the movable element 16 is moved toward the X1 direction.

In Document 3, one end of the movable element 16 on the X1 directionside is connected to a fixed surface 72 a of the valve body 12 or thelike on the X1 direction side through a spring member 70 a, while theother end of the movable element 16 on the X2 direction side isconnected to a fixed surface 72 b on the X2 direction side of the valvebody 12 or the like through a spring member 70 b. In this case, the twospring members 70 a, 70 b that are in a compressed state are providedbetween the movable element 16 and the fixed surfaces 72 a, 72 b. Thus,when the movable element 16 is at the neutral position shown in FIG. 7A,the spring members 70 a, 70 b on the both sides exert the elastic forcesindicated by the black arrows on the movable element 16. Since theseelastic forces are applied in mutually opposite directions, the forcesexerted on the movable element 16 are balanced.

As the movable element 16 slides from the neutral position toward the X2direction, as shown in FIG. 7B, the force exerted on the movable element16 from the spring member 70 a on the X1 direction side decreases, whilethe force exerted on the movable element 16 from the spring member 70 bon the X2 direction side increases. Further, when the movable element 16slides from the neutral position toward the X1 direction, as shown inFIG. 7C, the force exerted on the movable element 16 from the springmember 70 a on the X1 direction side increases, while the force exertedon the movable element 16 from the spring member 70 b on the X2direction side decreases.

FIG. 8 is a graph showing variations of the forces exerted on themovable element 16 from the two spring members 70 a, 70 b with respectto the position of the movable element 16. In FIG. 8, the broken linerepresents the force exerted on the movable element 16 from the springmember 70 a on the X1 direction side, the dot-and-dash line representsthe force exerted on the movable element 16 from the spring member 70 bon the X2 direction side, and the solid line represents the total forceexerted on the movable element 16. As shown in FIG. 8, the total forceexerted on the movable element 16 varies linearly with respect to theposition of the movable element 16, wherein no force is exerted on themovable element 16 at the neutral position (0). That is, the forcetoward the negative direction (X1 direction) increases as the movableelement 16 slides toward the X2 direction, while the force toward thepositive direction (X2 direction) increases as the movable element 16slides toward the X1 direction.

FIG. 9A to FIG. 11 show a case of a servo valve in Document 4.

FIG. 9A to FIG. 9C schematically show the construction of this servovalve. FIG. 9A shows the neutral position, FIG. 9B shows a case that amovable element 16 is moved toward the X2 direction, and FIG. 9C shows acase that the movable element 16 is moved toward the X1 direction.Document 4 differs from the servo valve 10 (refer to FIG. 1 to FIG. 6)according to the present embodiment in that one spring member 74 isdisposed in the X direction between one end portion of the movableelement 16 on the X1 direction side (spool 54) and the other end portionthereof on the X2 direction side (magnet portion 36) and that theopposite ends of the spring member 74 are fixed to two respective fixingportions 76 a, 76 b. Incidentally, the two fixing portions 76 a, 76 bare movable in the X direction.

In this case, at the neutral position shown in FIG. 9A, the two fixingportions 76 a, 76 b serving as spring seats are pressed by the elasticforce of the spring member 74 respectively on a fixed surface 78 a ofthe valve body 12 or the like on the X1 direction side and a fixedsurface 78 b thereof on the X2 direction side. Thus, movements of thetwo fixing portions 76 a, 76 b in the X direction are restrained. As aresult, even when the one end portion and the other end portion of themovable element 16 abut against the two fixing portions 76 a, 76 b, noload is exerted on the movable element 16 in the X direction.

Then, when the movable element 16 slides from the neutral positiontoward the X2 direction, the elastic force exerted on the movableelement 16 toward the negative direction (X1 direction) as indicated bythe black arrow in FIG. 9B increases. On the other hand, when themovable element 16 slides from the neutral position toward the X1direction, the elastic force exerted on the movable element 16 towardthe positive direction (the X2 direction) as indicated by the blackarrow in FIG. 9C increases.

For this reason, in Document 4, since as shown in FIG. 11, initial loadstoward the positive direction and the negative direction are balancedwith each other at the neutral position (0), the total load exerted onthe movable element 16 becomes zero ideally. On the other hand, as themovable element 16 moves further away from the neutral position, theforce exerted on the movable element 16 increases toward the positivedirection or the negative direction.

However, if the spool 54 of the movable element 16 has looseness orbacklash caused by the dimensional tolerance or the like, as shown inFIG. 10A and FIG. 10B, gaps W are produced between the fixing portions76 a, 76 b and the fixed surfaces 78 a, 78 b at the neutral position, orgaps W are produced between the fixing portions 76 a, 76 b and themovable element 16. Thus, the movable element 16 is liable to bedisplaced in the X direction at the neutral position. As a result, theactual characteristic of the force exerted on the movable element 16 isaffected by the gaps W, and as indicated by the solid line in FIG. 11,becomes a characteristic in which the initial loads are generated atpositions deviated from the neutral position. Consequently, in the servovalve of Document 4, it is impossible to control the position of themovable element 16 precisely.

FIG. 12 is a graph showing a relationship between the position of amovable element 16 and the force exerted on the movable element 16 in aservo valve in Document 5.

In this servo valve, the magnetic force of a permanent magnet is used asa restoring force for the movable element 16 (spool 54). Thus, as shownin FIG. 12, the force exerted on the movable element 16 has a linearcharacteristic where the force increases toward the positive directionor the negative direction as the movable element 16 moves farther fromthe neutral position. Thus, since the force of a magnetic spring aroundthe neutral position becomes weak, the movable element 16 is liable tobe displaced due to an impact or vibration from the outside.

FIG. 13 is a graph which, concerning a force (restoring force toward theneutral position) exerted on the movable element 16, shows the result ofthe present embodiment (solid line) shown in FIG. 4 together with theresults of Documents 3 to 5 shown in FIG. 8, FIG. 11 and FIG. 12(Document 3: dot-and-dash line, Document 4: broken line, and Document 5:two-dot chain line). As shown in FIG. 13, in Documents 3 to 5, themovable element 16 is displaced when an impact or vibration isexternally applied to the servo valve in the vicinity of the neutralposition, or the movable element 16 cannot be controlled precisely dueto backlash of the movable element 16.

In contrast, in the present embodiment, the force exerted on the movableelement 16 is kept at a predetermined value even when the position ofthe movable element 16 is changed toward the positive direction or thenegative direction. Thus, the position of the movable element 16 can becontrolled precisely in comparison with the cases of Documents 3 to 5.

4. MODIFICATIONS OF THE PRESENT EMBODIMENT

Next, modifications of the servo valve 10 according to the presentinvention will be described.

In the present embodiment, the closed center servo valve 10 has beendescribed in which, as shown in FIG. the connections of flow passagesbetween the ports 24 are blocked at the neutral position. The presentinvention is not limited to the aforementioned description and may be anexhaust center servo valve in which output ports communicate withexhaust ports at the neutral position, or a pressure center servo valvein which output ports communicate with a supply port at the neutralposition.

Further, in the present embodiment, description has been made regardingthe servo valve 10 having the movable portion (magnet portion 36) of themovable magnet type, as shown in FIG. 1. The servo valve 10 according tothe present invention is not limited to the aforementioned descriptionand may be a servo valve having a movable portion 80 of a movable coiltype shown in FIG. 14A to FIG. 14C or a servo valve having a movableportion 82 of a movable iron core type shown in FIG. 15A to FIG. 15C.Incidentally, FIG. 14A to FIG. 15C schematically show the constructionsof respective drive units 18.

In the case of FIG. 14A to FIG. 14C, the drive unit 18 of the servovalve 10 (refer to FIG. 1) has a tubular yoke 84 connected to the endportion of the valve body 12 on the X2 direction side, two permanentmagnets 86 a, 86 b provided at respective opposite ends of the yoke 84in the X direction so as to protrude inward, and a movable portion 80provided in the X direction inside the yoke 84 and constituting aportion of the movable element 16. The movable portion 80 has an ironcore 88 disposed substantially coaxially with the central axis 14 insidethe yoke 84 so as to face the yoke 84, and a coil 90 wound around theiron core 88. In this case, a protruding portion 88 a protruding towardthe yoke 84 is formed on a center portion of the iron core 88, and thecoil 90 is arranged on the iron core 88 by being wound around theprotruding portion 88 a.

FIG. 14A shows positioning of the drive unit 18 at the neutral position.In this case, it is desirable that the center position of the protrudingportion 88 a of the iron core 88 in the X direction be substantially inagreement with a middle position between the two permanent magnets 86 a,86 b. Further, it is desirable that the yoke 84 be provided so as tocover the movable portion 80 within a moving range of the movableportion 80.

Here, as one example, a case will be described that the movable element16 (refer to FIG. 1) including the movable portion 80 is slid toward theX1 direction. In a case where the two permanent magnets 86 a, 86 b aremagnetized as shown in FIG. 14A, the magnetic flux from the permanentmagnet 86 a forms a magnetic path which passes through the yoke 84, theprotruding portion 88 a and the like. Thus, a magnetic attractive forceindicated by the black arrow in FIG. 14B is generated due to themagnetic flux at the movable portion 80. As a result, the magneticattractive force causes a thrust force to be generated toward the X1direction indicated by the outlined arrow, whereby the movable element16 is slid toward the X1 direction against the first elastic force ofthe first elastic portion 62 toward the X2 direction (see FIG. 2C).

Further, as shown in FIG. 14C, when the magnetic flux is generatedaround the movable portion 80 by energization to the coil 90, themagnetic flux forms a magnetic path passing through the iron core 88including the protruding portion 88 a and the like, and thus, theprotruding portion 88 a is magnetized to be S-pole. Therefore, themagnetic attractive force exerted on the movable portion 80 furtherincreases, and the thrust force toward the X1 direction increasesaccordingly. Consequently, the movable element 16 is easily slid towardthe X1 direction (refer to FIG. 2C).

On the other hand, when the energization to the coil 90 is discontinuedin a case that the movable element 16 is moved toward the X1 direction,the magnetic attractive force decreases. Thus, the first elastic forceserves as a restoring force toward the neutral position, and hence, themovable element 16 can be returned to the neutral position shown in FIG.2A along the X2 direction.

Further, in the case of FIG. 15A to FIG. 15C, the drive unit 18 of theservo valve 10 has a tubular yoke 92 connected to the end portion of thevalve body 12 (see FIG. 1) toward the X2 direction, a permanent magnet94 provided on a center portion of the yoke 92 in the X direction, acoil 96 provided in the X direction inside the yoke 92 so as to face thepermanent magnet 94, and an iron core 98 (a movable portion 82 of themovable iron core type) provided in the X direction inside the yoke 92so as to penetrate through the coil 96 and which constitutes a portionof the movable element 16. In this case, the coil 96 is disposedsubstantially coaxially with the central axis 14. Further, the iron core98 is disposed substantially coaxially with the central axis 14 so as topenetrate through a hollow portion at the center of the coil 96.

FIG. 15A shows positioning of the drive unit 18 at the neutral position.In this case, it is desirable that the center position of the permanentmagnet 94 (the center position of the yoke 92), the position of the coil96 and the center position of the iron core 98 in the X direction besubstantially in agreement with each other.

Here, as one example, a case will be described that the movable element16 (see FIG. 1) including the iron core 98 is slid toward the X1direction. If the permanent magnet 94 is magnetized as shown in FIG.15A, magnetic flux from the permanent magnet 94 forms a magnetic pathpassing through a protruding portion 92 a (the yoke 92), the iron core98 and the like. Thus, a magnetic attractive force arising from themagnetic flux indicated by the black arrow in FIG. 155 is generated atthe iron core 98. As a result, the magnetic attractive force causes athrust force to be generated toward the X1 direction indicated by theoutlined arrow, whereby the movable element 16 is slid toward the X1direction against the first elastic force of the first elastic portion62 toward the X2 direction (see FIG. 20).

Further, as shown in FIG. 15C, when magnetic flux is generated aroundthe iron core 98 by energization to the coil 96, this magnetic fluxforms a magnetic path passing through the iron core 98, the yoke 92including the protruding portion 92 a and the like, and thus, theprotruding portions 92 a, 92 b and the both ends of the iron core 98 areeach magnetized to N-pole or S-pole. Therefore, the magnetic attractiveforce exerted on the iron core 98 further increases, and accordingly thethrust force toward the X1 direction increases. As a result, the movableelement 16 is easily slid toward the X1 direction (see FIG. 2C).

On the other hand, when the energization to the coil 96 is discontinuedin a case that the movable element 16 is moved toward the X1 direction,the magnetic attractive force decreases. Thus, the first elastic forceserves as a restoring force toward the neutral position, and thus, themovable element 16 can be returned to the neutral position shown in FIG.2A along the X2 direction.

Further, it is possible to construct the servo valve 10 according to thepresent embodiment into a modification as shown in FIG. 16. Thismodification differs from the construction shown in FIG. 1 in that oneend of a first elastic portion 62 is fixed to an end portion of thespool 54 on the X1 direction side while the other end of the firstelastic portion 62 is fixed to the end cover 22 and that the first fixedportion 56 is omitted. Thus, the second elastic portion 64 only isconnected to the connecting portion 58. In this case as well, the firstelastic portion 62 presses the movable element 16 including the spool 54toward the X2 direction. Thus, the modification is able to perform thesame operation as that in the construction shown in FIG. 1.

FIG. 17 is a graph showing a relationship between the position of themovable element 16 and the force exerted on the movable element 16 inthe modification shown in FIG. 16. In this case, a resultant force ofthe first elastic force and the second elastic force is a spring force,and a resultant force of the spring force and a magnetic attractiveforce is a restoring force. As mentioned before, the modification shownin FIG. 16 performs the same operation as the construction shown inFIG. 1. For this reason, also in FIG. 17, the restoring force is thesame as that shown in FIG. 4. Accordingly, also in the modificationshown in FIG. 16, it is possible to improve the controllability of theposition of the movable element 16.

5. EFFECTS OF PRESENT EMBODIMENT

As described above, in the servo valve 10 according to the presentembodiment, the first elastic portion 62 and the second elastic portion64 have the elastic forces (the first elastic force and the secondelastic force) that are applied in mutually different directions alongthe X direction.

Thus, at the neutral position of the movable element 16, the connectingportion 58 is pressed against a portion of the valve body 12 that facesthe drive unit 18 (i.e., the step portion 66 on the X1 direction side inthe large diameter portion 20 b) and a portion of the movable element 16that faces the drive unit 18 (i.e., the spool 54 on the X1 directionside in the large diameter portion 20 b). Therefore, since theconnecting portion 58 is restrained from moving toward the X1 direction,the position of the second elastic portion 64 is restrained in theinterior (the large diameter portion 20 b of the hole portion 20) of thevalve body 12. As a result, the second elastic force is not exerted onthe movable element 16, and thus, the movable element 16 is positionedon the neutral position at which the movable element 16 abuts againstthe connecting portion 58.

Next, when the movable element 16 is slid toward the drive unit 18 side(toward the X2 direction) by driving of the drive unit 18, the movableelement 16 is slid together with the connecting portion 58 toward the X2direction against the second elastic force. In this case, when drivingof the drive unit 18 is stopped, the second elastic force serves as arestoring force, whereby the connecting portion 58 and the movableelement 16 are returned to the neutral position along the X1 direction.

On the other hand, when the movable element 16 is slid in a directionaway from the drive unit 18 (toward the X1 direction) by driving of thedrive unit 18, the movable element 16 is slid toward the X1 directionagainst the first elastic force with the connecting portion 58 abuttingagainst the step portion 66. In this case, when driving of the driveunit 18 is stopped, the first elastic force serves as a restoring force,whereby the movable element 16 is returned to the neutral positiontoward the X2 direction.

Accordingly, in the present embodiment, even in any of the case that themovable element 16 moves toward the X2 direction and the case that themovable element 16 moves toward the X1 direction, it is possible tostably perform the positioning control (the opening control of theplurality of ports 24) of the movable element 16 with respect to theneutral position. As a result, it is possible to realize the servo valve10 having a satisfactory function of closed center, exhaust center orpressure center.

In this case, as with the construction shown in FIG. one end of thefirst elastic portion 62 is fixed to the first fixed portion 56 on thedrive unit 18 side of the movable element 16, one end of the secondelastic portion 64 is fixed to the second fixed portion 60 on the driveunit 18 side of the valve body 12, and the other end of the firstelastic portion 62 and the other end of the second elastic portion 64are connected to the connecting portion 58. Thus, with a simpleconstruction, it is possible to improve controllability of positioningthe movable element 16 with respect to the neutral position.

Further, in the servo valve 10, the drive unit 18 has the movableportion of the movable magnet type (the magnet portion 36 shown in FIG.1 and FIG. 3A to FIG. 3C), the movable portion 80 of the movable coiltype (refer to FIG. 14A to FIG. 14C), or the movable portion 82 (theiron core 98) of the movable iron core type (refer to FIG. 15A to FIG.15C). Thus, it is possible to easily slide the movable element 16 in theX direction. Like this, even in any case of the movable magnet type, themovable coil type and the movable iron core type, it is possible toimprove controllability in positioning the movable element 16.

Further, in the present embodiment, it is possible to adjust therestoring force for returning the movable element 16 to the neutralposition by equilibrating the magnetic attractive force exerted on themovable element 16 from the drive unit 18 with the first elastic forceor the second elastic force. Thus, it is possible to improve thepositioning control of the movable element 16. In particular, if themagnetic attractive force is balanced with the first elastic force orthe second elastic force to thereby adjust the restoring force so thatthe restoring force has a fixed value regardless of the position of themovable element 16 in the X direction, it is possible to further improvethe positioning control of the movable element 16.

Here, if the servo valve 10 has the magnet portion 36, the magnetic fluxis generated around the magnet portion 36 by energization to the coil34, and the magnetic attractive force arising from the magnetic flux isapplied on the magnet portion 36. As a result, it is possible to slidethe movable element 16 including the magnet portion 36 in the Xdirection against the first elastic portion 62 or the second elasticportion 64. That is, the drive unit 18 functions as a linear motor formoving the magnet portion 36 in the X direction. Thus, since it ispossible to easily control positioning of the movable element 16, it ispossible to improve the responsiveness of the servo valve 10 withrespect to an input signal supplied from the outside to the coil 34.

Further, since the first protruding portion 40 a or the secondprotruding portion 40 b constitutes a portion of the magnetic path ofthe magnetic flux when the coil 34 is energized, the magnet portion 36is moved in the X direction, and the magnetic attractive force becomeslarger as the magnet portion 36 comes closer to the first protrudingportion 40 a or the second protruding portion 40 b. Further, byequilibrating the magnetic attractive force with the first elastic forceor the second elastic force, as shown in FIG. 4 and FIG. 13, it ispossible to keep the restoring force for returning the movable element16 to the neutral position constant regardless of the position of themovable element 16. Accordingly, it is possible to improve thecontrollability of the servo valve 10 (the positioning control of themovable element 16, the responsiveness of the servo valve 10).

In this case, if the coil 34 is provided between the first protrudingportion 40 a and the second protruding portion 40 b and if the positionof the magnet portion 36 in the X direction and the position of the coil34 in the X direction are set to be substantially the same when themovable element 16 is at the neutral position, then it is possible tofurther improve controllability of the servo valve 10.

Further, if the first yoke 32 is connected to the valve body 12 so as tocover the magnet portion 36 within the moving range within which themagnet portion 36 is moved in the X direction together with sliding ofthe movable element 16, it is possible to further improve thecontrollability of the servo valve 10.

Further, since the first yoke 32 is constructed by arranging the sideyoke 32 a and the outer yoke 32 b with the coil 34 interposedtherebetween in the X direction, it is possible to improve theassembling performance of the servo valve 10.

Furthermore, the magnet portion 36 is composed of the first permanentmagnet 36 a and the second permanent magnet 36 b arranged in the Xdirection and magnetized in the X direction, and the second yoke 36 cinterposed between the first permanent magnet 36 a and the secondpermanent magnet 36 b. Thus, at the time of energization to the coil 34,the magnetic flux generated around the magnet portion 36 passes throughthe second yoke 36 c, and as a result, a large thrust force arising fromthe magnetic attractive force is generated at the magnet portion 36 inthe X direction. Therefore, it is possible to easily slide the movableelement 16 in the X direction against the first elastic force or thesecond elastic force.

In this case, when the first permanent magnet 36 a and the secondpermanent magnet 36 b are magnetized in mutually different magnetizationdirections, it is possible to easily slide the movable element 16 towardthe X1 direction or the X2 direction.

Further, since the servo valve 10 is further provided with the magneticsensor 48 disposed adjacent to the magnet portion 36 in the X directionand configured to detect the magnetic flux from the magnet portion 36,it is possible to easily grasp the position of the movable element 16relative to the neutral position from change in the magnetic fluxdetected by the magnetic sensor 48. Consequently, it is possible toperform a suitable servo control by adjusting an input signal suppliedto the coil 34 depending on the position of the movable element 16.

Further, since the servo valve 10 having the movable portion 80 of themovable coil type or the movable portion 82 (the iron core 98) of themovable iron core type is also able to slide the movable element 16 inthe X direction by the magnetic attractive force as in the case of theservo valve 10 having the aforementioned movable portion (the magnetportion 36) of the movable magnet type, it is possible to easily performthe positioning control of the movable element 16. In this case as well,it is possible to improve the responsiveness of the servo valve 10.

Furthermore, as in the modification shown in FIG. 16, also in the casethat one end of the first elastic portion 62 is fixed to the spool 54 ofthe movable element 16, and the other end thereof is fixed to the endcover 22, while one end of the second elastic portion 64 is fixed to thesecond fixed portion 60 and the other end thereof is fixed to theconnecting portion 58, similarly to the structure shown in FIG. 1, it ispossible to improve, with a simple structure, the controllability inpositioning the movable element 16 with respect to the neutral position.

Furthermore, if the first elastic portion 62 and the second elasticportion 64 are spring members, it is possible to achieve reduced costsin the servo valve 10.

Obviously, the present invention is not limited to the foregoingembodiment and modifications, and it is a matter of course that variousconstructions can be effected thereto based on the contents of the abovedescription.

What is claimed is:
 1. A servo valve comprising: a body including aplurality of ports formed therein; a movable element disposed inside thebody in an axial direction of the body; and a drive unit connected tothe body in the axial direction and configured to slide the movableelement in the axial direction to thereby switch connections of flowpassages between the ports, the servo valve further comprising: a firstelastic portion extending in the axial direction inside the body on oneend portion side thereof and having a first elastic force to press themovable element toward the drive unit in the axial direction, the driveunit being connected to another end portion of the body; a secondelastic portion extending in the axial direction inside the body onanother end portion side thereof and having a second elastic force topress the movable element toward the one end portion side of the bodyalong the axial direction; and a connecting portion inside the body,wherein at a neutral position of the movable element at which driving ofthe drive unit is stopped, the connecting portion faces one end portionof the body and the movable element, wherein one end of the firstelastic portion is directly fixed to an end surface of an end portion ofthe movable element, wherein the end portion of the movable elementterminates at the end surface and does not extend into or around thefirst elastic portion; another end of the first elastic portion is fixedto an end cover that closes the one end portion of the body being apartfrom the drive unit; one end of the second elastic portion is connectedto the connection portion; and another end of the second elastic portionis fixed to the another end portion side of the body and the drivingunit.
 2. The servo valve according to claim 1, wherein: the drive unitincludes a tubular body comprising a magnetic body and connected to thebody in the axial direction, and a movable portion provided inside thetubular body and forming a portion of the movable element, the movableportion including a movable magnet, a movable coil or a movable ironcore; and by moving the movable portion in the axial direction, themovable element including the movable portion is slid in the axialdirection.
 3. The servo valve according to claim 2, wherein: the driveunit has a first yoke, which serves as the tubular body, connected tothe body in the axial direction, a coil disposed inside the first yoke,and a magnet portion, which serves as the movable portion, providedinside the first yoke so as to face the coil; and a magnetic attractiveforce exerted on the magnet portion due to energization to the coilcauses the movable element to slide in the axial direction.
 4. The servovalve according to claim 3, wherein: protruding portions protrudinginward of the first yoke are provided respectively at one end side andanother end side of the first yoke in the axial direction; and at theneutral position at which energization to the coil is stopped, themagnet portion is positioned between the two protruding portions.
 5. Theservo valve according to claim 4, wherein: the coil is provided betweenthe two protruding portions inside the first yoke; and when the movableelement is at the neutral position, the magnet portion and the coil arelocated at a substantially same position in the axial direction.
 6. Theservo valve according to claim
 3. wherein: the first yoke is connectedto the body so as to cover the magnet portion within a moving rangewithin which the magnet portion is moved in the axial direction bysliding of the movable element.
 7. The servo valve according to claim 3,wherein: the first yoke comprises two yokes arranged so as to interposethe coil between the two yokes in the axial direction.
 8. The servovalve according to claim 3, wherein the magnet portion comprises: twopermanent magnets arranged in the axial direction and magnetized in theaxial direction: and a second yoke interposed between the two permanentmagnets.
 9. The servo valve according to claim 8, wherein: the twopermanent magnets are magnetized in mutually different directions. 10.The servo valve according to claim 3, wherein: a sleeve provided withopenings communicating with the respective ports is disposed inside thebody: the movable element includes the magnet portion, a spool disposedinside the sleeve and movable in the axial direction, and a shaftconnecting the magnet portion and the spool in the axial direction; anannular fixed portion is provided inside the tubular body and on a firstyoke side, the annular fixed portion being fixed to the tubular body andthe first yoke, wherein the shaft penetrates through the annular fixedportion, and the another end of the second elastic portion is fixed tothe annular fixed portion; the connecting portion is an annular memberconfigured to, inside the body, abut against the spool and against theinner peripheral surface of the body, the shaft penetrating through theconnecting portion.
 11. The servo valve according to claim 3, furthercomprising: a sensor disposed adjacent to the magnet portion in theaxial direction and configured to detect magnetic flux.
 12. The servovalve according to claim 2, wherein: the drive unit includes a yoke,which serves as the tubular body, connected to the body in the axialdirection, two permanent magnets provided respectively at opposite endsof the yoke in the axial direction, an iron core provided inside theyoke so as to face the yoke, and a coil wound around the iron core; themovable portion comprises the iron core and the coil; and the movableelement is slid in the axial direction by a magnetic attractive forceincluding at least one of a force acting between the two permanentmagnets and the iron core, and a force exerted on the movable portiondue to enemization to the coil.
 13. The servo valve according to claim2, wherein: the drive unit includes a yoke, which serves as the tubularbody, connected to the body in the axial direction, a permanent magnetprovided at a central portion of the yoke in the axial direction, a coilprovided inside the yoke in the axial direction so as to face thepermanent magnet, and an iron core, which serves as the movable portion,provided inside the yoke in the axial direction; and the movable elementis slid in the axial direction by a magnetic attractive force includingat least one of a force acting between opposite ends of the yoke and theiron core, and a force exerted on the iron core due to energization tothe coil.
 14. The servo valve according to claim 1, wherein: the firstelastic portion and the second elastic portion are spring members. 15.The servo valve according to claim 1, wherein: a restoring force toreturn the movable element to the neutral position is adjusted bybalancing a force exerted on the movable element from the drive unitwith the first elastic force or the second elastic force.